To produce oil or gas, a well is drilled into a subterranean formation that is an oil or gas reservoir.
Well Servicing and Well Fluids
Generally, well services include a wide variety of operations that may be performed in oil, gas, geothermal, or water wells, such as drilling, cementing, completion, and intervention. Well services are designed to facilitate or enhance the production of desirable fluids such as oil or gas from or through a subterranean formation. A well service usually involves introducing a well fluid into a well.
Drilling is the process of drilling the wellbore. After a portion of the wellbore is drilled, sections of steel pipe, referred to as casing, which are slightly smaller in diameter than the borehole, are placed in at least the uppermost portions of the wellbore. The casing provides structural integrity to the newly drilled borehole.
Cementing is a common well operation. For example, hydraulic cement compositions can be used in cementing operations in which a string of pipe, such as casing or liner, is cemented in a wellbore. The cement stabilizes the pipe in the wellbore and prevents undesirable migration of fluids along the annulus between the wellbore and the outside of the casing or liner from one zone along the wellbore to the next. Where the wellbore penetrates into a hydrocarbon-bearing zone of a subterranean formation, the casing can later be perforated to allow fluid communication between the zone and the wellbore. The cemented casing also enables subsequent or remedial separation or isolation of one or more production zones of the wellbore, for example, by using downhole tools such as packers or plugs, or by using other techniques, such as forming sand plugs or placing cement in the perforations. Hydraulic cement compositions can also be utilized in intervention operations, such as in plugging highly permeable zones or fractures in zones that may be producing too much water, plugging cracks or holes in pipe strings, and the like.
While drilling an oil or gas well, a drilling fluid is circulated downhole through a drillpipe to a drill bit at the downhole end, out through the drill bit into the wellbore, and then back uphole to the surface through the annular path between the tubular drillpipe and the borehole. The purpose of the drilling fluid is to maintain hydrostatic pressure in the wellbore, lubricate the drill string, and carry rock cuttings out of the wellbore.
The drilling fluid can be water-based or oil-based. Oil-based fluids tend to have better lubricating properties than water-based fluids, nevertheless, other factors can mitigate in favor of using a water-based drilling fluid. Such factors may include but not limited to presence of water-swellable formations, need for a thin but a strong and impermeable filtercake, temperature stability, corrosion resistance, stuck pipe prevention, contamination resistance and production protection.
Cementing and Hydraulic Cement Compositions
Hydraulic cement is a material that when mixed with water hardens or sets over time because of a chemical reaction with the water. The cement composition sets by a hydration process, passing through a gel phase to solid phase. Because this is a chemical reaction with water, hydraulic cement is capable of setting even under water.
The hydraulic cement, water, and any other components are mixed to form a hydraulic cement composition in fluid form. The hydraulic cement composition is pumped as a fluid (typically in the form of suspension or slurry) into a desired location in the wellbore. For example, in cementing a casing or liner, the hydraulic cement composition is pumped into the annular space between the exterior surfaces of a pipe string and the borehole (that is, the wall of the wellbore). The hydraulic cement composition should be a fluid for a sufficient time before setting to allow for pumping the composition into the wellbore and for placement in a desired downhole location in the well. The cement composition is allowed time to set in the annular space, thereby forming an annular sheath of hardened, substantially impermeable cement. The hardened cement supports and positions the pipe string in the wellbore and fills the annular space between the exterior surfaces of the pipe string and the borehole of the wellbore.
Wettability and Wetting of Solid Surfaces
The wettability of a solid surface or a film on a solid surface can impact various well applications. For example, an oleaginous film on a metal surface of a tubular or a rock material of a subterranean formation can affect bonding of hydraulic cement to the surface. The wettability of rock or the wetting of the rock can affect the flow of a fluid through the matrix of rock of a subterranean formation.
Wettability involves the contact between a liquid and a solid surface, resulting from the intermolecular interactions when the two different phases are brought together. In general, the degree of wetting (wettability) depends on a force balance between adhesive forces between the liquid and solid surface and cohesive forces of the liquid (i.e., surface tensions). Adhesive forces between a liquid and solid cause a liquid drop to spread across the surface. Cohesive forces within the liquid cause the drop to ball up and avoid contact with the surface.
A measurement of the degree of wettability of a material is the contact angle, the angle at which the liquid interface meets the dry solid interface. If the wettability is very favorable to the liquid, the contact angle will be low, and the fluid will spread to cover or “wet” a larger area of the solid surface. If the wettability is unfavorable, the contact angle will be high, and the fluid will form a compact, self-contained droplet on the solid surface. If the contact angle of a water droplet on a solid surface is less than 90°, the surface may be said to be “water-wettable” (and inverse proportionally, probably not oil-wettable); whereas if the contact angle of an oil droplet on a solid surface is less than 90°, the surface may be said to be “oil-wettable” (and inverse proportionally, not water-wettable.
As used herein, a wet or wetted surface or the wetting of a surface may refer to a liquid phase that is directly in contact with and adhered to the surface of a solid body. For example, the liquid phase can be an oleaginous film on the surface of a metallic tubular or the face of a borehole in the material of a subterranean formation.
Some well fluids can form such a film or layer on a downhole surface, which can have undesirable effects. The fluid (or a liquid component of the fluid) can form a film or layer on the surface, which can act as a physical barrier between the material of the underlying solid body and a fluid adjacent to the surface of the solid body. In effect, such a film presents a different wettability characteristic than the material of the underlying solid body. For example, an oleaginous film on the surface of a metal tubular blocks water from wetting the underlying surface, which would otherwise be water-wettable.
A metallic surface of a downhole tubular is typically both water wettable and oil wettable. If first wetted with an oleaginous film, however, the oleaginous film on the metallic surface blocks the metal surface from being wettable with a water-based fluid.
Wetting of Tubulars and Formation Surfaces for Cementing
Hydraulic cement compositions do not bond well to oil-wetted surfaces. After drilling a wellbore with an oil-based drilling mud, the surfaces of tubulars and the formation in the wellbore may become oil-wetted with an oleaginous film. It is necessary to remove the film on the solid surfaces from being oil-wetted with such a film to improve cement bonding. The primary method of cleaning of oil-wet surfaces in the wellbore is through chemical and mechanical action through application of wall-shear applied at the surface due to pumping of a spacer fluid past the surface before the introduction of cement.
In a case where complete surface wetting with water is not achieved prior to placing cement in the desired zone of interest, only partial bonding of the surfaces with cement is obtained. Because of this incomplete surface bonding, there is a proportional decrease in the shear bond strength of the interface between the set cement sheath and the formation/tubular surfaces and premature interfacial de-bonding might occur under the loads experienced during the course of the well operations. This may have unwanted consequences such as interzonal communication, loss of production, and sustained casing pressure. Any of these can be detrimental to the safety and economics of hydrocarbon production from the well.
It would be highly desirable in well operations to have apparatuses and methods for determining wettability at subterranean wellbore temperature, pressure, shear, and other conditions. Applications include, for example, the designing of spacer or inverter fluids and determining the field-operational parameters for wellbore cleanout and fluid separation prior to cementing operations in a well.